We aimed to validate the technique of breath counting during overbreathing in revealing ictal impairment of cognition during brief generalized 3 Hz spike and slow wave discharges. A retrospective study of 66 patients with video-electroencephalographic documentation of typical absence seizures revealed 8 patients in whom there was no clinical suspicion of absences but who had brief (less than 4 seconds) generalized 3 Hz spike-wave discharges. The only clinical manifestations of the absences were abnormalities in breath counting during overbreathing. These consisted of slowing of speech, delay in counting, repetition of numbers and counting out of sequence combined with delay. Abnormalities were more likely to occur during longer discharges. Detection of these abnormalities led to important changes in the classification of the patients' epilepsies and in the recommendations for treatment. The technique of breath counting during overbreathing is a simple practical and reliable method for detecting even mild cognitive impairment during 3 Hz spike-wave discharges.